1. Field of the Invention
The present invention relates to an optical element and an optical pickup device used therewith, and more particularly, to the optical element having a phase difference adjusting function and the optical pickup device used therewith.
2. Description of the Related Art
Optical pickup devices are widely used in order to write/read information to/from optical recording media such as CD (Compact Disk) and DVD (Digital Versatile Disk). In the optical pickup device, after P-polarized light or S-polarized light emitted from a light source is converted into circularly-polarized light by a quarter wavelength plate, an advancing direction of the light is changed by a reflecting mirror or the like and the light is condensed onto an optical recording medium by an objective lens. After the circularly-polarized light which is reflected by the optical recording medium is converted into S-polarized light or P-polarized light by the quarter wavelength plate, the S or P-polarized light is detected as a light signal by a light detector.
U.S. Published Patent Application No. US2002/093902A1 discloses that in the case where two light sources are used, after optical paths from the respective light sources are synthesized by a dichroic prism, linearly polarized light from the light sources is converted into circularly-polarized light by a quarter wavelength plate.
In the optical pickup device disclosed in the above publication, light converted into circularly-polarized light is reflected by a deflection prism. A phase difference is generated between a P-polarized light component and an S-polarized light component at the time of reflection, and the reflected light may become elliptically-polarized light. Since the elliptically-polarized light does not become linearly-polarized light even when it transmits through the quarter wavelength plate, signal strength to the detector is decreased or return light to the light source is generated, resulting in a problem that laser oscillation becomes unstable. Further, not only when the light is reflected but also when the light transmits through an optical element, a phase difference is generally generated between the P-polarized light component and the S-polarized light component. For this reason, the optical element, which reflects or transmits the light converted into the circularly-polarized light, is required not only to satisfy spectroscopic properties such as reflectance and transmittance but also to prevent the generation of a phase difference.
Since the wavelength of light sources is being shortened in recent years, a so-called blue laser, whose laser beam has a wavelength of 400's nm which is shorter than a laser beam having a wavelength of 600's nm to be currently and mainly used in DVD, is being put into practical use. For this reason, optical pickup devices which are mounted with three light sources with wavelength of 700's nm (for CD), wavelength of 600's nm (for DVD) and wavelength of 400's nm (for example, for BD (Blu-ray Disk) or for HD DVD (High Definition DVD)) are being currently developed.
In the design of optical thin films which are formed on substrates of optical elements, however, when an optical pickup device, which satisfies the spectroscopic properties such as reflectance and transmittance for such light having three wavelength bands and simultaneously prevents the generation of a phase difference due to reflection and transmission, is tried to be obtained, it is difficult to design a satisfactory thin film constitution because of too many restriction conditions. For this reason, the thin film has been designed so that one of the spectroscopic properties and the phase difference properties is prioritized and the other is sacrificed.